The invention relates to systems that distribute time information to multiple remote locations and, more particularly, the invention relates to a method and apparatus for distributing a time and frequency information to multiple remotely located high definition television studios.
A modern television studio contains a plurality of studio components that must be accurately time synchronized to facilitate switching between television signals produced by various sources within the studio. In a conventional NTSC system, the timing reference for both analog and digital signals within the studio are based on the notion of xe2x80x9csystem genlocksxe2x80x9d, in which all studio components are locked to a studio reference via a dedicated timing reference cable. As such, each and every component of an NTSC studio has a signaling cable as well as a genlock timing reference cable connected to it. Within a genlock system, frames of video information are moved synchronously between components. The genlock signal is instrumental in ensuring that frame synchronization is maintained between all the components of a studio. However, in a situation where data is passed between components using compressed digital formats, such as is used in high definition television (HDTV) broadcast studio, the genlock system is ineffective at maintaining synchronization between components with respect to the compressed digital signals.
In a HDTV studio, the various components within the studio are physically connected to each other through a central switch called a studio data router. This router is responsible for processing not only the video bitstreams that are coupled to the studio, but also to provide command and control services to the studio and any other services that are required by the studio. The low-level transport for data within the studio is asynchronous transfer mode (ATM) which is well suited for providing multiple data streams to a single point. Heretofore, there has not been developed a timing and frequency distribution system that would be equivalent to the system genlock of an NTSC studio for a HDTV studio. Since many studios share data and information through satellite communications and cable communication systems, it would be advantageous to have a global sense of time to be distributed not only to components within the studio, but also to all studios within a network such that retiming of signals at any studio location is not necessary prior to broadcast.
Therefore, a need exists in the art for a method and apparatus for distributing a global sense of time (and frequency) to a plurality of studios such that the studios can naturally time lock all of the studio components to the global time reference.
The disadvantages associated with the prior art are overcome by a method and apparatus for distributing a reference frequency and a reference time to a plurality of high definition television (HDTV) studios such that the studios can then use the reference frequency and time to lock their studio components to a global frequency and time reference. The invention includes various embodiments for facilitating the distribution of time and frequency depending upon the type of digital network that is used for distributing the television signals within a studio components within a studio. The first embodiment is useful in a system that uses a synchronous optical network (SONET) for distributing television signals to a plurality of studio components within a studio. This embodiment extracts a reference frequency signal from the SONET signals such that all the components in the studio that are equipped with this embodiment can be locked to the SONET frequency. Within this form of signal distribution, the SONET path and line overhead bytes can be used to distribute a global sense of time of day information to the studio components.
Another embodiment of the invention is a second method and apparatus for distributing a global sense of time of day information to multiple components within a studio. In this embodiment, the program clock reference within an MPEG transport stream is used for the distribution of time of day information to various studios and their studio components. Since MPEG transport streams are used in all forms of digital television information distribution, this embodiment is useful in SONET and non-SONET based distribution systems.
The last embodiment of the invention uses the internet engineering task force and network time protocol for distribution of time of day information to the studios and their respective components.